SSL Certificate for client's e-mail

I have a server running with WHM/cPanel which I use to sell shared hosting accounts (cPanel) to clients.

We allow clients to create email accounts via cPanel.
When setting up e-mail accounts on their computers, the clients receive a message that the SSL certificate of the server is untrusted.

My question is, how can we overcome this problem?
I do not know much about SSL, and therefore I am bit lost with finding out which one has to be used to resolve this issue.
Is there a certificate that I can install on the server as a root, and this one will cover all clients accounts for e-mail?
Every client of course has a different domain.
Is there a certificate that will make e-mails from the server SSL trusted?